The field of the present invention is apparatus for radiographic image acquisition and material composition analysis, and more particularly, apparatus employing x-ray reflective structures for narrow bandwidth and multiple energy x-ray imaging.
In recent years substantial efforts have been directed toward the development of x-ray optics for XUV and soft x-ray imaging applications such as x-ray microscopes, telescopes, and the imaging of hot plasmas (Underwood, J. Attwood D, Physics Today, p. 44, April, 1984). Due to such efforts, a certain amount of attention has been given to multilayer mirrors. For example, multilayer mirror structures have been considered as an alternative to conventional grazing incidence metal mirrors. A comparison between metal and multilayer mirrors shows that the multilayer offers several advantages:
(1) The x-ray angle of incidence can be increased beyond the critical angle.
(2) The multilayer is capable of rejecting (exhibiting low reflectivity) energies outside the narrow bandwidth reflectivity peak.
(3) The thickness of individual layers in a multilayer structure can be varied as well as the materials comprising the layers. For a given angle of incidence .theta., the reflectivity bandwidth can be altered.
(4) Multilayers can be deposited using vapor, sputtering, laser, molecular beam epitaxy or other deposition techniques on fabricated substrates of various geometric shapes.
As early as 1940, DuMond and Yountz (J. Applied Physics 11: 357, 1940) reported on a vapor deposited multilayer structure comprised of alternating gold and copper layers with a 100 .ANG. (angstrom) period. Since the latter part of the 1970's, it has been possible to form quality multilayer structures with small (less than 10-20 .ANG.) layer thicknesses using a variety of materials (e.g. W, ReW, Au Hf, Pt, Ta, Fe, Mo, Be, B, C, Si) and deposition techniques. Typically, multilayers are deposited on carefully polished substrates (silicon, float glass, mica, etc.) or diamond turned metal substrates since the roughness of individual layers tends to emulate that of the substrates surface.
Multilayer structures have also been used as wide-band monochromators, replacing very narrow bandwidth Bragg x-ray crystals such as silicon and it may be convenient to speak of a multilayer structure as a synthetic x-ray crystal.
The ability to deposit a high (H) atomic number material (H=W, ReW, Au, Hf, Pt, Ta, Mo, Fe, etc.) and a low (L) atomic number material (L=B, Be, C, Si, etc.) as alternating layers of adjustable thickness permits the design of a multilayer structure which offers high peak reflectivity and a much larger bandwidth than a Bragg x-ray crystal material such as silicon. Depending on the reflected energy and multilayer design, the bandwidth may be larger by a factor of several hundreds to several thousands. The simplest multilayer designs range from Bragg-like (dH&lt;&lt;dL) to quarter wave stack (dH=dL) where dH and dL are the layer thicknesses of the high and low atomic number materials, respectively, that make up the multilayer structure (Spiller, AIP Proceedings No. 75: 124, 1981).
The position of the primary reflectivity peak (m=1) is also influenced by the layer pair thickness and can be crudely calculated using the Bragg equation ( m.lambda.=2d sin .theta., m=1, 2, 3, . . . , n) where is the order, .lambda.=wavelength, d=dH+dL=the period, and .theta.=the angle of incidence measured from the surface). Similarly, the intensity of the (higher order) reflectivity peaks is influenced by the layer pair ratio dH/dL. A sinusoidal variation in the thickness dH and dL has been suggested as a means of minimizing reflectivity for orders m&gt;1 (Bilderback, Nucl. Instr. Methods Vol. 208: 251, 1983). A multilayer based on a graded layer thickness design has been employed as a means of increasing mirror collection efficiency for a diverging source (Nagel, Nucl. Instr. Methods Vol. 195: 63, 1982).
While numerous patents have been issued on methods of preparing multilayer structures, they are largely directed to materials which are useful for electro-optic applications (e.g. Dingle et al., U.S. Pat. No. 4,261,771). Specific multilayer designs which could be of value for x-ray imaging at radiographic energies between 10-100 KeV are not emphasized. In the field of medical radiography, Imaging and tissue composition analysis problems encountered using x-ray spectra available with W-anode (and in some cases Mo-anode) x-ray tubes are severe. The wide bandwidth bremsstrahlung x-ray spectrum (W-anode) used in many radiological examinations imposes undesirable limitations on the information content of the recorded image. The effect of passing the x-ray beam through the patient is to harden the beam and so reduce image contrast. In many instances the radiologist compensates for this loss of image contrast or material composition information by increasing the image x-ray statistics and/or the invasiveness of the procedure. Additional restrictions on attainable image contrast are imposed by the dynamic range and energy resolution capabilities of the x-ray receptor. Various approaches which have been employed to minimize the spectral bandwidth problem include:
(1) Using the characteristic emission spectrum from a dedicated molybdenum (Mo)-anode tube for mammography. The gamma emitters .sup.153 Gd and .sup.125 I have been used as sources in rectilinear scanners and CT units for bone absorptometry and densitometry.
(2) Testing special filter materials for mammography (Rh, Pd, Mo), and angiography (I, Cs). Narrow bandwidth spectra require heavy filtration, resulting in severe tube heat loading since the tube voltage must be restricted. This is due to the relatively low absorption of the material for energies immediately below and well above the K-edge energy.
(3) Employing x-ray beams with distinctly different spectra. Dual energy imaging has been implemented in computed tomography and projection radiography with varying success. The high and low voltage x-ray beams requires extensive calibration and image subtraction entails computer-intensive calculations.
In addition to having undesirable emission spectra, conventional x-ray tubes used in medical radiography for area or slit scanning have additional limitations which conventional filtration materials cannot mitigate:
(1) Anode materials or combinations of anode materials (composite anodes) are extremely restricted. The two most common anode materials are Mo (mammography) and W (generally radiography). A much less common anode material (a composite) is a Mo-W alloy (used for mammography). The characteristic x-ray energies of these materials are substantially different and the anode is not typically used at a voltage where the emission lines of W are prominent. Using a composite anode comprised of materials whose characteristic x-rays energies are similar is unacceptable with conventional filter materials due to the relatively high transmission below the K-edge and the need to use a substantial thickness of filter material to minimize the higher energy emission lines which are very intense. Tube heating would be substantial.
(2) The small size of the x-ray tube focal spot (necessary to ensure adequate spatial resolution in the recorded projection image) places severe demands on the capacity of the tube to dissipate heat. Conventional W-anode tubes may have focal spots sizes of 1.5.times.1.5 mm while tubes used for mammography often have focal spots of 0.6 mm.times.0.6 mm and sometimes less than 0.3 mm.times.0.3 mm. Redistribution of the heat generated by the electron beam hitting the anode often involves rotating the anode at a high speed and possibly providing additional oil or water cooling. The permissible beam current (and thus x-ray fluence) decreases as the focal spot size decreases. The anode material has a limited ability to dissipate heat quickly in the immediate area of the focal spot. The total time a unit area of the anode is continuously bombarded by the electron beam is dependent on the rotation rate. The time between bombardments depends on the radius of the rotating anode. Enlarging the anode (while maintaining the rotation speed) increases the total heat capacity of the tube, but not the upper limit on instantaneous tube output which is related to the power handling capabilities (the melting point) of the anode material. The total fluence can be increased by increasing the focal spot size. The available fluence may be of concern if patient motion reduces image quality (that is, long image acquisition times are not acceptable). In addition, employing materials such as Ag in an anode for an application where a small focal spot is required may be questionable since the heating limitations are more severe for Ag than for Mo. A material such as Ag might be used in an anode if the focal spot could be extended to cover a larger area and the tube voltage could be increased to a higher, more efficient operating level.
(3) Beam divergence, which results in variable magnification from the center to the edge of the image and also for internal structures within an object that are different distances from the source. Maintaining an adequate distance between object and source helps to limit this effect. A slit scan system which uses a single slit for each distinct x-ray source provides a fairly constant magnification in the scan direction (which is typically perpendicular to the length of the collimating slit) while allowing the beam magnification to vary continuously from the center to edge of the image along the length of the slit.
A slit scanning system which uses a single diverging x-ray source and multiple slits must deal with the variable magnification problem. Multiple slits in a scanning unit are useful for reducing the tube heating and typically reducing scan times. Multiple slit scanning devices often maintain a fixed source, stationary patient and frequently a stationary area detector while allowing the slits to move in such a fashion as to maintain exposure uniformity (Barnes et al. Med. Phys. 6(3): 197, 1979; King et al. Med. Phys. 10(1): 4, 1983; Rudin et al. Med. Phys. 9(3): 385, 1982). However, variable magnification is maintained over the entire image for conventional multiple slit scanning devices. Consider, instead, scanning with a multiple slit configuration while creating relative linear motion between the source+slits and subject, and an area detector which functions as an image integrator. As with other multiple slit scan designs, the total exposure for a segment of the image represents the sum of contributions from several slits. Care must be taken so as to avoid extensive overlapping of individual slit projects which have noticeably different projection angles. This will tend to degrade image resolution in the direction of scan. Even if each slit has its own set of detectors, the final image would be comprised of a number of strip segments which were acquired at various projection angles with respect to the scanning direction. Conventional x-ray beam filtration materials (Al, Cu, etc.) will not affect the divergent nature of the focal spot (the magnification properties of the imaging system).
Conventional single and multiple slit scanning systems also encounter problems when utilized for dual energy studies. Because the tube voltage must be switched between low and high values for best results (minimal overlap of the two spectra), images must be acquired at two voltages (either line by line or entire images) to obtain acceptable data for tissue composition analysis. In addition, these dual energy scanning techniques require extensive beam calibration using a phantom comprised of many combinations of (typically) lucite and Aluminum (Macovski et al., Comput. Biol. Med. 6: 325 1976; Lehmann et al., SPIE Vol. 314: 143, 1981; Lehmann et al., Med. Phys. 8(51): 659, 1981). Conventional x-ray tube slit scan systems do not permit a single line to be imaged simultaneously at two distinct narrow bandwidth energies or to acquire, simultaneously, two lines at distinctly different energies of narrow bandwidth and at approximately the same projection angle with a single source. Maintaining the same projection angle for the same line acquired at two different energies is very important since dual energy "subtraction" is done by image processing of data from both images.
Slit scanning systems which utilize a radionuclide source (which includes simple projection and CT scanning) in place of an x-ray tube have been employed for tissue composition analysis application in medical radiography such as the detection of certain bone diseases (bone mineral loss). Such systems have similar restrictions:
(1) The selection of potential radiation sources is limited since many radionuclides emit several energies of x-ray and/or gamma ray which would limit energy resolution. Introducing K-edge material filters is often an unsatisfactory means of limiting the bandwidth since the choice of K-edge filters is limited, they attenuate both below and above the K-edge, and for high energy x-rays most K-edge filters would have to be very thick to provide acceptable attenuation.
(2) A scanning system employing a radionuclide source often requires a long time period to scan the patient or object. This is because radionuclide sources (as with other materials) exhibit self-absorption which has an exponential dependence on material thickness. A small focal spot source of even low intensity is difficult to produce and requires a large quantity of the radionuclide. The need for a small source size parallels the problem of x-ray tube focal spot size (maintaining adequate spatial resolution).